Is it forbidden or reprehensible to love a girl without the knowledge of her family, and what is preferable in that regard?

If your inclination towards the girl is for the purpose of marriage, free from prohibitions, and the girl is religious, of good character, and wears the hijab, then there is no harm in it, provided you propose to her immediately. If they agree, then praise be to Allah. If they refuse, then turn your heart away from her and supplicate for her success. As for continuing to be attached, meeting, and communicating with her without the knowledge of her family, that is unlawful and a betrayal.